The present invention relates to an improved covering fabric for a damping form roller of an offset printing machine. More particularly, the invention relates to the covering fabric in which drafted polyurethane fibers fixed to water-soluble fibers are used as ground yarn.
In offset printing it is necessary to wet the surface of a printing plate with water before the application of oil-based printing inks. For such a purpose, a damping form roller covered by fabric has been used. Therefore the covering fabric must have a sufficient water absorbing capacity and furthermore must be firmly mounted on the damping form roller. Heretofore there is conventionally used a covering fabric which is manufactured by knitting fibers into a cylindrical form having piles on the outer surface, where the piles may be cut or uncut piles. A non-woven cylindrical covering sleeve consisting of hydrophilic materials has also been used. In order to facilitate the covering operation and to mount firmly on the roller, the above-mentioned covering fabric or sleeve can be manufactured having a diameter of larger than that of the roller and, after covering of the roller, the covering fabric or sleeve is shrunk, where lightly heat-treated and non-acetalized multifilament polyvinyl alcohol fibers are used as a water-shrinkable ground yarn of the covering fabric (U.S. Pat. No. 3,926,701) and polyvinyl alcohol fibers, rayon fibers and polypyrrolidone fibers are used as a water-shrinkable substrate of the covering sleeve (U.S. Pat. No. 3,229,351). Such covering fabric and sleeve are firmly mounted on the roller at the starting of use, however, it happens that the ground yarns and the substrate become gradually weak as these are always in wet state and, consequently the covering fabric or sleeve is twisted or bulged. These disadvantages become more remarkable at a high speed offset printing.
In order to avoid such disadvantages, there is proposed a covering fabric using, as the ground yarn, polyurethane fibers as free length coiled by cotton fibers. Such a covering fabric is manufactured having a diameter of smaller than that of the roller. In mounting this covering fabric, it is stretched and then fitted on the roller by shrinkage of polyurethane fibers. This covering fabric is, however, poor in contractibility and it tends to slip on the roller. Thus, this covering fabric must have a diameter of very smaller than that of the roller in order to mount firmly on the roller. But, it is difficult to mount such a small covering fabric on the roller.